Washington Geology
Total Page：16
File Type：pdf, Size：1020Kb
WASHINGTON GEOLOGIC NEWSLETTER '· Volume 15 Number 3 July 1987 Washington State Department of Natural Resources Division of Geology and Earth Resources Pa rt of bu i Id ing in Pou I sbo on an ancient , deep-seated sI ide • Reactivation of the sI ide in 1974 severely damaged the building and adjacent streets and utilities. (see article, p. 3) IN THIS ISSUE . Soil bluff failure • • • • • 3 Oi I and gas activity • • • • • • 12 Division open file geologic maps available • ••• 16 WASHINGTON TIMBER, FISH AND WILDLIFE GEOLOGIC by NEWSLETTER Raymond Lasmanis, State Geologist Leaders from the timber industry, environmental groups, Indian tribes, and the The Washington Geologic Newsletter is published quarterly by the Division of Geology and Earth Resources, Department of Natural Resources. The newsletter is state Departments of Fisheries, Game, free upon request. The Division also publishes bulletins, information circulars, re Ecology, and Natural Resources have been ports of investigations, and geologic maps. A list of these publications will be sent upon request. meeting since August 1986 to reach a mutually satisfactory agreement on forest DEPARTMENT Brian J. Boyle practices regulations and forest management. OF Commissioner of Public Lands A comprehensive package has been developed NATURAL Art Stearns RESOURCES Supervisor by this Timber, Fish and Wildlife group (TFW) that calls for significant changes in DIVISION OF Raymond Lasmanis the manner in which forest practices are GEOLOGY AND State Geologist evaluated and regulated and natural resources EARTH RESOURCES J. Eric Schuster Asst. State Geologist a re managed. Of the the eleven key components of the new management system under the TFW Geologists Bonnie B. Bunning Weldon W. Rau (Olympia) Michael A. Korosec Katherine M. Reed program, forest practices on unstable slopes William S. Lingley, Jr. Henry W. Schasse have been identified as a major issue because Robert L. (Josh) Logan Gerald W. Thorsen harvesting timber and \.\Orking on such slopes William M. Phillips Timothy J . Walsh can, in some instances, accelerate natural Geologists Nancy L. Joseph Keith L. Stoffel instability processes and impact fish, wild..! (Spokane) I ife, water, cultural and archaeological Librarian Connie J . Manson resources, public capital improvements, and off-site property. Management decisions re Research Technician Arnold Bowman garding the potential impacts of such timber Editor Katherine M. Reed harvests can be imp roved by ( 1 ) using more Cartographers Nancy E. Herman complete technical and scientific data and Donald W. Hiller information; (2) having technical specialists Keith G. Ikerd on the ground to assess impacts and offer Word Processor Operator J. C. Armbruster appropriate management alternatives; and (3) Administrative Assistant Susan P. Davis defining ways to measure the effectiveness of Clerical Staff Loretta M. Andrake state-of-the-a rt management practices. Willa Bonaparte Resolution of the unstable slopes issue Joy Fleenor wi 11 require investment by state government, Regulatory Clerical Staff Barbara A. Preston industry, and others in imp roving the (continued on page 15 ) MAILING ADDRESSES Martin Way Main Office: Department of Natural Resources To Division of Geology and Earth Resources Interstate 5 Mail Stop PY.12 Olympia, WA 98504 : MAIN OFFICE : Phone: (206) 459-6372 .,;; :Geology and Earth Resources: o:: : 4224-6th Ave. S.E. : Field Office: Department of Natural Resources South ~ ~ : Rowe Six, Building One 1 0 Division of Geology and Earth Resources -~ L----------,.-, ---------- -- --"' u Spokane County Agricultural Center Sound :,::: Arctic ..........,- N. 222 Havana Center lii Circle l!__J Paulson's Spokane, WA 99202 D St. Martin's Phone: (509) 456-3255 Mall ~D 6th Ave. S.E. College iJi Albertsons NOTE: Publications available from Olympia address only. D 2 SOIL BLUFFS + RAIN = SLIDE HAZARDS by Gerald W. Thorsen INTRODUCTION confined to either coastal areas or western Washington. Many miles of such bluffs in • Lands I ide season• in the Puget Lowland eastern Washington are now sliding, primarily normally occurs from November through April. as a result of reservoir drawdown and During that period, familiar names commonly irrigation. Here, areas relatively dry for reappear in the press. The Alki, Queen thousands of years are now being subjected to Anne, and Magnolia districts of Seattle are • artificial precipitation•. The resulting in mentioned in the news nearly every winter. creases in soil water locally have profound This repetition is because natural landslides effects on bluff stability. u sua I ly occur where they have occur red be fore. In spite of such warnings, there is a SETTING THE STACE variety of reasons to crowd development against or even to occupy slide hazard areas. Repeated continental glaciation punctuated If these reasons are important enough, the by long periods of floodplain, delta, and lake risks and/or extra costs may be acceptable. deposition has left thick sections of un Probably the commonest reasons for in consolidated materials over the bedrock of the dividuals choosing to live with the slide Puget Lowland. These materials, "soil" to hazards of Puget Lowland bluffs are the view from the top ( Fig. 1) or, in the case of waterfront property, access to water from the bottom. Where industry is concerned, there can be even more compelling reasons to accept such hazards. For example, a rail road must have a relatively level, albeit narrow route. A beach can meet both cri teria (Fig. 2). Unfortunately, in the Puget Lowland, beaches a re commonly backed by unstable bluffs. Such compromises of location and risk must also be made with other transportation systems. The Seattle freeway was bui It, at staggering cost, through a zone of unstable bluffs and dormant landslides. Because of the stabi I ity prob Iems , the area had been relatively unoccupied. The political and eco nomic decisions for selecting such a route were thus considered justifiable. Similarly, some of our most popular shoreline parks ( such as Kopachuck, Camano, Sequim Bay, and Golden Gardens) probably exist largely because the areas were too unstable for other Figure 1 .--Demand for view property, development • such as this area overlooking the Strait of This article is a discussion of Puget Juan de Fuca, continues to increase along Lowland bluffs and the more common forms of with the Puget Sound area's population. landslides affecting them. Most examples are Here, local collapse of somewhat cohesive from coastal bluffs, as they are among the sandy soil causes it to break up and flow best exposed and most active. Stabi I ity to the beach. Winter storms remove the problems associated with steep bluffs in un dry-flow deposits, re-exposing the toe of consolidated sediments are, of course, not the bluff to wave erosion. 3 Figure 2.--A large landslide complex south of Edmonds. The activities of railroad track maintenance crews have replaced wave erosion as the means of removing: mud f lows at the toe of the s I ide • the geologist or engineer, are hundreds or even thousands of feet thick in places. Following the sculpting and compaction by the last continental ice sheet to occupy the Lowland, wave, as \\<ell as modern and Pleistocene stream erosion (Figs. 3 and 4) have cut hundreds of miles of steep bluffs into these sediments. Many of these miles are in or near population centers. Thus, the pres sure for residential development is great and is increasing. Of these bluffs, probably those along the marine waters of the Puget Lowland have been the most systematically studied.. The slope stability maps of the Coastal Zone Atlas ( Washington Dept. of Ecology, 1978-80) show more than 660 miles of these bluffs as "unstable". Included in this category are approximately 71 miles of recently active landslides and 65 miles of old, apparently dormant slides. Both figures are, no doubt, Figure 3 .--These homes are part of a understated. This is because recent slides development overlooking the floodplain of narrower than about 200 feet were difficult the Stillaguamish River near Interstate to show at the map scale used and some old Highway 5. The till bluff is the scarp of slides ~re probably not recognized. Marine a large landslide {Minard, 1985) probably bluffs may not be fully represenitative of the triggered by the river undercutting the bluffs fringing floodplains or glacial melt bluff. water channels, but they vivid Iv display the 4 are discussed here only because the slide type often provides clues to the nature of the hazard. Slumps A common Puget Lowland slide form is the upper bluff slump. In most of the places where these occur, the lower bluff is usually very compact silt or "blue clay" that does not fail at all. Instead, the slumped upper bank material, once it is disturbed, turns into mud and flows down and over the steep but sti 11 intact lower bluff (Fig. 6). This mode of failure leaves a distinctive landform, the mid-bluff bench, along the top of the lower unfailed silt or clay. Figure 4 .--Lake Kapowsin and ground fog The bluff below the bench is very steep, mark this sinuous glacial meltwater commonly exceeding 60°. The bench itself channel. Remnants of soil bluffs fringing tends to have a hummocky surf ace, and the such long abandoned channels remain along steepness of the slope above it increases the fronts of the Cascade foothills. upward, approaching ve rt ica I at the sea rp of the slump. This bench may not be easily range of stabi I ity problems that exist recognized if the last movement was not throughout the Low Iand. recent and if the vegetative cover is dense. Some of the best examples of upper bluff LANDSLIDES slumping in the Seattle area are in the Alki area, Fort Lawton, and Golden Gardens. In Gulleying, dry ravel, and other forms of the last instance, the park area itself has essentially particle-by-particle erosion are apparently not had a serious slope failure in important bluff-modifying processes.